Hispano-Moresque ceramic tiles from the Monastery of Santa Clara-a-Velha (Coimbra, Portugal)

A group of late 15th-early 16th century Hispano-Moresque glazed tiles from the Monastery of Santa Clara-a-Velha, in Coimbra, were, for the first time, characterised chemically and morphologically. Since the prevailing idea among art historians in Portugal is to judge the Hispano-Moresque tile heritage as Spanish production, the findings concerning technological processes were compared with the Islamic technology in the Iberian Peninsula and latter Hispano-Moresque in Spain. This study is the first analytical indicator of a production technology with some differences from the Hispano-Moresque workshops (such as Seville, Toledo, etc.) and points out to a possible local production. Five different coloured glazes were identified: white, blue, green, amber and black, all displaying high-lead content, as expected for this type of ceramics. Tin oxide was identified in high contents (7-14wt.%) in white and blue glazes, its crystals homogenously distributed in also very homogeneous glazes, showing similarities with an Islamic glazing technology. On the other hand, Ca-rich thick glaze-ceramic interfaces were observed, with many mineral inclusions (wollastonite (CaSiO₃) and also K-feldspars (general formula KAlSi₃O₈), showing a higher resemblance with a later Hispano-Moresque technology. Other compounds were also identified from reactions involving the colour compound: malayaite (CaSnOSiO₄), bustamite (CaMnSi₂O₆), braunite ((Mn²⁺, Mn³⁺)₆O₈SiO₄), andradite (Ca₃Fe₂Si₃O₁₂), magnesioferrite (MgFe₂O₄) and a nickel ferrite (NiFe₂O₄). The chemical composition of this glaze-ceramic interface suggests firing temperatures between 950°C and 1000°C and its thickness implies a single-fire process. The chemical characterisation does not suggest different recipes or different firing processes for cuerda seca and arista tiles.